1. Field of the Invention
The present invention relates to a radar having a sampling gate circuit for a video signal.
2. Description of the Prior Art
It has been known to use a pulse radar for detecting the distance to a target from the time difference between a transmission wave and a received wave by transmitting a pulse wave under constant periodic motion and receiving the reflected wave from the target.
One embodiment of the conventional radar will be illustrated by referring to FIG. 1. In FIG. 1, a reference pulse oscillating circuit 1 generates a reference pulse under constant periodic motion to feed it to a microwave head 2 and a saw tooth wave generator 7. The microwave head 2 is triggered by the reference pulse to generate a pulse microwave shown in FIG. 2a to radiate through an antenna 3 to space. The microwave is reflected by the target to return to the antenna 3 so as to be received as shown in FIG. 2b. The frequency conversion of the microwave is performed by the microwave head 2 to transmit an intermediate frequency signal to an intermediate frequency amplifying circuit 4. The signal amplified in the intermediate frequency amplifying circuit is detected by a detecting circuit 5 to give the signal shown in FIG. 2c. The detected output is amplified by a video signal amplifying circuit 6 and is then fed to a cathode-ray tube 9.
The saw tooth wave generating circuit 7 is triggered by reference pulse oscillating circuit 1 to generate the saw tooth wave of FIG. 2d. The saw tooth wave is voltage-amplified or current-amplified by sweep circuit 8 and is then utilized to operate cathode-ray tube 9.
When the detecting signal of FIG. 2c is a signal for vertical deflection and the saw tooth wave signal of FIG. 2d is a signal for horizontal deflection, the signal intensity in the vertical direction and the time in the horizontal direction is shown on the picture plate of the cathode-ray tube as shown in FIG. 3.
The distance to the target can be given from the equation EQU L = Ct.sub.1 /2 [m] (1)
wherein C designates the velocity of the wave in the horizontal time base, [m/sec]; L designates a distance to the target [m] and t.sub.1 designates the difference between the transmission wave and the received wave [sec].
However, in the conventional radar, the output given by detecting the intermediate frequency signal is displayed on the picture plate of the cathode-ray tube 9. Accordingly, in order to accurately amplify the pulse signal waveform of the reflected wave, the following frequency band width B is required as a characteristic of the video signal amplifying circuit 6. EQU B = 1.2/t.sub.o .about. 1.5t.sub.o [1/sec.] (2)
wherein t.sub.o designates the transmission pulse width of the microwave [sec.] The transmission pulse width t.sub.o in the case of a near detecting distance of 50 [m] .about. 2 [Km] should be less than 0.1 [.mu.s] when the distance resolution is less than 15 m.
Accordingly, a broad band width from low frequency to about 15 [MHz] is required for the video signal amplifying circuit 6 from the equation: EQU B = 1.5/0.1 .times. 10.sup.-.sup.6 [1/sec.] (3).
For the cathode-ray tube 9, a similar band width is required for displaying the pulse waveform. Accordingly, a cathode-ray tube having a quick frequency response and a broad band is required. Moreover, the video signal amplifying circuit and the cathode-ray tube have broad bands whereby the pulse noise shown in FIG. 2 is also easily amplified. The noise is also displayed on the picture plate of the cathode-ray tube 9 as shown in FIG. 3 so as to cause erroneous measurement.